Resistors having passages extending through a body of ceramic resistance material of positive temperature coefficient of resistivity have been proposed for use as fluid heaters. Such heaters are safe and self-regulating and are adapted to generate high volume outputs of heated air for a hair dryer or the like or to safely and efficiently heat the air-fuel mixture being supplied to an automobile engine to assure early volatilization of the fuel before the mixture is furnished to the engine. In one such device, ohmic contact coatings are formed on the inner walls of the resistor body passages and selected groups of the contact coatings are interconnected so that electrical current can be directed through the thin webs of resistance material between contacts of opposite polarity in adjacent body passages, thereby to generate a large amount of heat for efficient transfer to a fluid directed through the passages. In one particularly advantageous structure of that type, interconnection of the desired groups of ohmic contact coatings is facilitated by molding thin lands of the resistance material around alternate ones of the passages at one end of the resistor body. Similar lands are molded around the ends of the other passages at the opposite end of the resistor. Coatings of electrically conductive interconnecting materials are then applied to the ends of the resistor so that all of the ohmic contacts are connected together at each end of the body. The body ends are then abraded for conveniently removing the interconnection coating material from the tips of the lands, thereby to leave two groups of ohmic contacts interconnected by the conductive material which remains on the ends of the body around the sides of the lands or between the lands.
Frequently, however, such resistor heater devices are difficult and expensive to manufacture. For example, in the last noted structure, the lands tend to chip during handling either before application of the interconnection coating material or during the abrading steps or the like with the result that some of the devices are found to be defective. The provision of the lands also tends to complicate the molding of the resistor body. In addition, providing the lands necessitates the use of relatively thicker passage walls than would otherwise be required, thereby resulting in higher resistance in the webs of resistance material as the resistor heater is initially energized. As will be understood, the use of thicker walls also requires the use of a larger volume of the resistance material than might otherwise be necessary for achieving a selected heating capacity.
It is an object of this invention to provide a novel and improved multi-passage resistor; to provide such a resistor adapted for use as a fluid heater which is formed of a multipassaged body of ceramic resistance material of positive temperature coefficient of resistivity having ohmic contact coatings formed on the inner walls of the body passages and having novel and improved means electrically interconnecting selected groups of the contacts with improved convenience and reliability; to provide such resistor heater devices which embody relatively lesser volumes of resistance material and which display relatively lower resistance when initially energized; to provide such devices which are of a compact and rugged structure; and to provide such devices which are relatively inexpensive to manufacture.